Hi Gin Gin,
That last one shows the Viewport. The default Viewport Rebnder Settings are a lot worse (Noise Threshold of 0.1) than those for actual Renders. You can’t compare Viewport without Denoise with Render with Denoise.
I’d use a lot of Samples and maybe disable the Noise Threshold, or set it quite low (definitely not higher than 0.01).
The main problem is Denoise and Animation. There are Addons that can do so-called Temporal Denoising (Denoising for Animations). For instance:
https://pidgeontools.gumroad.com/l/PidgeonToolBag
(it has a free version, so you can try it out.)
